Morphological and Molecular Evidence for Infrageneric Relationships in Blastus (Melastomataceae)

Blastus Lour. is a taxonomically complex genus of Sonerileae (Melastomataceae), with unresolved infrageneric relationships and controversial boundaries among several species and varieties. In this study, we integrated morphological evidence with a broadly sampled internal transcribed spacer (ITS) dataset, an expanded nuclear ribosomal DNA (nrDNA) dataset, plastome data, and genomic single-nucleotide polymorphism (SNP) data to evaluate the phylogenetic relationships and classification within Blastus. The broadly sampled ITS and expanded nrDNA analyses supported the monophyly of the sampled Blastus accessions but provided limited resolution of relationships among closely related taxa. The plastome analysis did not recover the sampled Blastus accessions as monophyletic and revealed marked cytonuclear phylogenetic incongruence. The genomic SNP analysis strongly supported the monophyly of the sampled Blastus accessions and recovered the sampled members of the terminal inflorescence group as a monophyletic clade. However, the sampled members of the traditional axillary inflorescence group were not monophyletic because Blastus borneensis was recovered as sister to the terminal inflorescence clade. Within the terminal inflorescence clade, the sampled members of the B. pauciflorus and B. cavaleriei groups formed two strongly supported sister lineages. Morphological comparisons showed that inflorescence position and architecture, the distribution of yellow glands on the abaxial leaf surfaces, and calyx lobe morphology generally corresponded to the principal lineages recovered in the SNP tree, whereas indumentum characters varied considerably among individuals and populations. We therefore recommend retaining the axillary and terminal inflorescence groups as descriptive morphological categories and treating the B. pauciflorus and B. cavaleriei groups as two informal working subdivisions within the terminal inflorescence group. The broad circumscription of B. pauciflorus adopted in the Flora of China requires reassessment, whereas the boundaries of B. longiflorus var. apricus and B. dunnianus require broader population sampling before formal taxonomic changes are proposed.